She seems harmless enough now, the elderly figure in a dressing gown peering round the door to her flat.

Erna Wallisch, an 85-year-old grandmother, rarely ventures out, spending her days drinking coffee and being cared for by her family.

But the image she presents belies a dark past which has put her seventh on the Simon Wiesenthal Centre's list of most-wanted Nazi war criminals still at large.

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As a young woman during the Second World War, Mrs Wallisch inspired terror in those she helped to guard in Nazi concentration camps.

Some former prisoners say she beat inmates to death.

One, who told how she attacked a young boy when she was pregnant, said: "The sweating, breathless face of that monster is something I will never forget."

Mrs Wallisch, who was born in eastern Germany, lives in a small apartment on the banks of the Danube in Vienna where she was tracked down by the British historian and author Guy Walters. Despite her 'most-wanted' status, her name is printed on the bell-push.

When found by Mr Walters, Mrs Wallisch refused to comment on his investigation into her past as a brutal concentration camp guard.

Neighbours said they knew nothing about her history. Most told Mr Walters she should be left alone, rather than brought to justice. "It's all in the past and should be forgotten," said one. "People should learn to forgive."

The Austrian government is refusing to prosecute Mrs Wallisch, saying it is too late.

"I do not find this attitude surprising," said Mr Walters, who is writing a book called Hunting Evil about the escape and pursuit of Nazi war criminals.

"For too long, the Austrians have been unacceptably lenient with these evil men and women in their midst. I suspect their reluctance to confront these criminals is because it would only highlight the extent of Austrian complicity with Nazism."

It is not known if Mrs Wallisch's family who bring her groceries and sit chatting to her are, like her neighbours, unaware of her past.

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Mrs Wallisch, the daughter of a postal clerk, was born Erna Pfannenstiel in 1922. She joined the Nazi party as a teenager and became a guard at the Ravensbruck women's concentration camp near Berlin where British SOE agent Violette Szabo was among the tens of thousands murdered.

Mrs Wallisch was later transferred to the Majdanek death camp near Lublin in Poland where she was based between October 1942 and January 1944. The testimony of at least four former inmates has been gathered in an attempt to bring justice for her victims.

They allege that Mrs Wallisch used 'violence and illegal threats for reasons of race and nationality, against women and children weakened physically and psychologically, from peoples within regions under civil occupation . . . she treated them in an inhumane way.'

In Lublin, she met Georg Wallisch, a Nazi guard, whom she married in March 1944.

Jadwiga Landowska, a former prisoner, recalled how the thenpregnant Mrs Wallisch beat people to death. "The pregnant Nazi monster woman who went crazy and attacked us did not appear among those tried after the war," she said.

"The pregnant one hit a young boy lying on the floor with something harder than a whip. Blood was pouring from his head and he gave no sign of life or reaction. The sweating, breathless face of that monster is something I will never forget."

It is not clear why Mrs Wallisch left Majdanek. Her file includes reports in which her superiors described her as an unruly and 'mouthy' person. It is known that there were plans to move her to Auschwitz.

The Simon Wiesenthal Center, named after a Holocaust survivor who dedicated the rest of his life to hunting Nazi war criminals, is based in Los Angeles.

Its director, Dr Efraim Zuroff, has demanded action over Mrs Wallisch from Austria's Justice Ministry. He argued that she had admitted participation in the mass murder of inmates at Majdanek and that a Polish survivor had testified that she was a sadist and had personally participated in selections for the gas chamber.

Officials say her crimes come under the statute of limitations and Austria cannot prosecute her.

The Justice Ministry said it is planning to commission a report on those still alive who worked at Majdanek. Mr Walters is unimpressed.

"Mrs Wallisch is an old woman and these reports always take time," he said. "It is highly likely that she will live out her days untroubled by the authorities. The problem is that too many in Austria believe these events belong in history books.

"This is nonsense. Mrs Wallisch is not history - she is still alive and it is our generation's responsibility to prosecute those whom she helped consign to history prematurely."